1. Field of the Invention
The present invention relates to a light beam deflector for deflecting a guided light beam in an optical waveguide. A surface elastic wave is generated in the optical waveguide and diffracts the guided light beam. More particularly this invention relates to a light beam deflector in which a light beam is deflected through a wide angle by being deflected a plurality of times with a plurality of surface elastic waves.
2. Description of the Prior Art
There is known a light beam deflector, as disclosed in Japanese Unexamined Patent Publication No. 61(1986)-183626, which comprises an optical waveguide made of a material capable of propagating a surface elastic wave therethrough. A light beam is applied to the optical waveguide and propagated therethrough as a guided wave. A surface elastic wave is produced in the optical waveguide and directed across the guided wave, and the guided wave is diffracted by the surface elastic wave through Bragg diffraction. In order continuously to vary the angle of diffraction (i.e. the angle of deflection) of the guided wave, the frequency of the surface elastic wave is continuously varied. A light beam deflector of this type is advantageous over a mechanical light beam deflector such as a galvanometer mirror or polygon mirror, an electrooptic deflector (EOD), and an acoustooptic deflector (AOD), since it may be small in size, light in weight and highly reliable as it has no mechanical movable parts.
However, a light beam deflector employing an optical waveguide has a problem in that it cannot achieve a large angle of deflection. More specifically, since the light beam deflecting angle is substantially proportional to the frequency of the surface elastic wave, if a large angle of deflection is to be obtained, then the frequency of the surface elastic wave has necessarily has to be increased to a very high value. Therefore, the frequency of the surface elastic wave would have to vary over a wide range. In addition, to meet the conditions for Bragg diffraction, the direction of travel of the surface elastic wave would have to be continuously steered to control the angle of incidence of the guided wave on the surface elastic wave.
To meet the above requirements, there has been proposed a light beam deflector as also disclosed in Japanese Unexamined Patent Publication No. 61(1986)-183626, which has a plurality of interdigital transducers (IDT) for generating respective surface elastic waves with frequencies which vary over different ranges. The IDTs are oriented such that they emit the surface elastic waves in different directions, and they are alternately switched into and out of operation.
This light beam deflector has a problem in that since the diffraction efficiency is lowered around the crossover frequency of the surface elastic waves generated by the IDTs, the intensity of the deflected light beam depends on the angle of deflection.
An IDT which deflects the light beam through a large angle must be arranged so as to be capable of producing a surface elastic wave of a very high frequency. This will be described below with reference to an example. If it is assumed that the angle of incidence of the guided wave on the surface elastic wave is .theta., then .delta., the angle of deflection of the guided wave due to an acoustooptic interaction between the surface elastic wave and the guided wave, is .delta.=2.theta., which is expressed as follows: ##EQU1## where .lambda. is the wavelength of the guided wave, Ne is the effective refractive index of the optical waveguide with respect to the guided wave, and .LAMBDA., f, v are the wavelength, frequency, and speed, respectively, of the surface elastic wave. Therefore, the deflection angle range .DELTA.(2.theta.) becomes: EQU .DELTA.(2.theta.)=.DELTA.f.multidot..lambda./Ne.multidot.v
If a deflection angle range .DELTA.(2.theta.)=10.degree. is to be obtained with .lambda.=0.78 .mu.m, Ne=2.2, and v=3500 m/s, for example, then the frequency range of the surface elastic wave, i.e., the range of high frequencies to be applied to the IDT must vary by .DELTA.f=1.72 GHz. If this frequency range is selected to be 1 octave so as not to be affected by secondary diffracted light, then the central frequency is f.sub.0 =2.57 GHz, and the maximum frequency is f.sub.2 =3.43 GHz. The wavelength .LAMBDA. of the surface elastic wave which is produced by the IDT and has the maximum frequency f.sub.2 becomes .LAMBDA.=1.02 .mu.m, and the line width W of the electrode fingers of the IDT becomes W=.LAMBDA./4=0.255 .mu.m.
With the conventional photolithographic and electron beam printing processes used for fabricating IDTs, the limits for the line widths at present are 0.8 .mu.m and 0.5 .mu.m, respectively. It is therefore impossible to fabricate an IDT having very small line widths as described above. Even if such a finely fabricated IDT could be produced in the future, it would be difficult and highly expensive to produce a driver for generating a frequency as high as 3.43 GHz, and it would be difficult to apply a high voltage to such an IDT. Moreover, if the frequency of the surface elastic wave is increased, as described above, the wavelength thereof is reduced, and hence the surface elastic wave is absorbed to a greater extent by the optical waveguide, resulting in a reduction in the diffraction efficiency.
IEEE Transactions on Circuits and Systems, vol. CAS-26, No. 12, p. 1072 [Guided-Wave Acoustooptic Bragg Modulators for Wide-Band Integrated Optic Communications and Signal Processing] by C. S. TSAI, does not disclose a light beam deflector in which a plurality of IDTs are switched into and out of operation, but does disclose a single IDT constructed as an IDT having arcuate electrode fingers, each having a continuously varying line width, which causes the frequency and the direction of travel of a surface elastic wave to vary continuously over a wide range. The disclosed arrangement eliminates the aforesaid problem of variation in the intensity of a light beam, which variation depends on the angle of deflection of the light beam, but still requires the surface elastic wave to have a high frequency.
The applicant has proposed a light beam deflector which can deflect a light beam through a wide angle and in which the intensity of the light beam does not vary and the frequency of a surface elastic wave is not set to a high value (see U.S. patent application No. 127,020).
The proposed light beam deflector, in which surface elastic waves diffract and deflect a guided wave traveling through an optical waveguide which is made of a material capable of propagating the surface elastic waves therethrough, includes:
first surface elastic wave generating means for generating in the optical waveguide a first surface elastic wave which travels across the light path of the guided wave and diffracts and deflects the first guided wave traveling along the light path; and
second surface elastic wave generating means for generating in the optical waveguide a second surface elastic wave which travels across the light path of the diffracted guided wave and diffracts and deflects the guided wave in a direction which amplifies the deflection thereof caused by the diffraction,
the first and second surface elastic wave generating means being arranged so as to continuously vary the frequencies and directions of the first and second surface elastic waves while meeting the conditions: EQU .sub.1 + .sub.1 = .sub.2 EQU .sub.2 + .sub.2 = .sub.3
where .sub.1 is the wave vector of the guided wave before it is diffracted by the first surface elastic wave, .sub.2 is the wave vector of the guided wave after it is diffracted by the first surface elastic wave, .sub.3 is the wave vector of the guided wave diffracted by the second surface elastic wave, and .sub.1, .sub.2 are the wave vectors of the first and second surface elastic waves.
Each of the first and second surface elastic wave generating means comprises a tilted-finger chirped interdigital transducer having electrode fingers spaced at distances or intervals which vary stepwise and oriented in directions which vary stepwise, and a driver for applying an alternating voltage having a continuously varying frequency to the tilted-finger chirped interdigital transducer.
Since the guided wave deflected by the first surface elastic wave is deflected again by the second surface elastic wave, the light beam deflector can provide a total deflection angle range which is wide even if each of the first and second surface elastic waves does not have a wide frequency range.
Three or more surface elastic waves may be propagated in a single optical waveguide so that a guided wave will be diffracted and deflected three or more times. According to such a modification, all of two adjacent surface elastic waves will diffract and deflect the guided wave and should be generated in the same manner as the first and second surface elastic waves referred to above. Thus, the guided wave can be deflected through a wider angle than if it is diffracted twice.
Generally, an IDT of the type described above and a driver (which comprises a high-frequency amplifier and a frequency sweeper) for applying an alternating voltage having a swept frequency to the IDT are employed as a means for generating a surface elastic wave. It is recognized that the generated surface elastic wave is subject to periodic intensity variations or fluctuations. More specifically, since the IDT and high-frequency amplifier are designed for use in a high-frequency range, their impedance is widely different from the impedance (i.e., of 50 .OMEGA.) of ordinary high-frequency systems. Because of their differing impedances, a large reflection is caused between a high-frequency cable and the IDT, and a reflected wave is sent back to the high-frequency amplifier. Inasmuch as the high-frequency amplifier is also designed for use in a high-frequency range, as described above, it reflects a wave having a power of several percent to several tens of percent of the power of the high-frequency wave, which propagates again toward the IDT. The reflected wave directed toward the IDT and a high-frequency wave (traveling wave) which is produced by the high-frequency amplifier interfere with each other, thereby varying the intensity of the high-frequency wave. Because the high-frequency cable has a constant length, the intensity of the high-frequency wave periodically cycles between high and low values as the frequency is swept in the high-frequency range.
When the intensity of the high-frequency wave applied to the IDT fluctuates, the intensity of the surface elastic wave generated by the IDT also varies, with the consequence that the efficiency with which the guided wave is diffracted by the surface elastic wave also vacillates. As a result, the quantity of the diffracted light beam, i.e., the intensity of a deflected light beam, varies. When the light beam deflector has a plurality of IDTs, as described above, the variation in the diffraction efficiency is amplified each time the guided wave is diffracted by a surface elastic wave, and the intensity of the deflected light beam tends to fluctuate to a large extent.